Birshot is for birds. You may get suitable results using a full choke & large birdshot @ close range but open choke riot guns, nope. With a spread of 1" per yard, that is a 5-6" pattern on a guys chest @ 20ft. If he is wearing a heavy leather coat, he is not suffering much from shot #6 or smaller.
I have tested this on a beef ribcage @ 20ft. The #6 shot did NOT penetrate thru the back side of the ribcage covered w/ 2 layers of denim. So serious SD/HD use starts w/ #4 buckshot & larger. Anything else is a risk you can take, but not me. If it's bad enough fo rme to be going to a SG, it's bad enough that the fight needs to stop right now. The ribcage on the left was hit W/ #4 buck, open choke 870. The right was #6 shot.